1. Field of the Invention
The present invention, an optical communication apparatus which transmits wavelength-division multiplexed signal light, relates to an optical communication apparatus whose operation is stabilized irrespective of presence/absence of input light or a modulated signal to be transmitted, as well as to an optical add/drop apparatus using such an optical communication apparatus as an addition apparatus.
2. Description of the Related Art
Ultra-long-distance and large-capacity optical communication apparatuses are now required to construct future multimedia networks. Concentrated studies are now being made of the wavelength-division multiplexing as a method for realizing large-capacity apparatuses in view of such advantages that it can effectively utilize a wide bandwidth and a large capacity of an optical fiber.
In particular, studies are in progress about an optical add/drop apparatuses of the wavelength-division multiplexing method and optical modulators used in the addition section of such an optical add/drop apparatus that is required in each node of the lightwave network.
In the Mach-Zehnder interferometer type optical modulators (hereinafter abbreviated as “MZ modulator”) that are used as optical modulators in conventional optical communication apparatuses, it is necessary to stabilize the output optical signal with respect to a variation and the variation with temperature and time. Japanese Patent Laid-Open No. 251815/1991 discloses an operating point control circuit for controlling the operating point of an MZ modulator intended for this purpose.
FIG. 20 is a block diagram of an MZ modulator having this conventional operating point control circuit.
As shown in FIG. 20, light exit from a light source 310 such as a laser diode (hereinafter abbreviated as “LD”) is entered to an MZ modulator 311. A modulation signal including information to be sent and a low-frequency signal of a predetermined frequency f0 that is outputted from a low-frequency oscillator 324 are inputted to a variable gain amplifier 313. The variable gain amplifier 313 superimposes the low-frequency signal of the predetermined frequency f0 on the modulation signal and outputs it, which is then inputted to one modulation-input terminal of the MZ modulator 311 via an amplifier 314 for obtaining a predetermined signal level and a coupling capacitor 315. A bias T circuit consisting of an inductor 316 and a capacitor 317 is connected to the other modulation-input terminal of the MZ modulator 311. The capacitor 317 is grounded via resistor 318. A portion consisting of the amplifier 314, the coupling capacitor 315, the bias T circuit, and the resistor 318 are equivalent to a drive circuit of the MZ modulator 311.
The MZ modulator 311 modulates light that is supplied from the light source 310 with a signal that is given by the drive circuit and outputs a resulting signal.
Part of an optical output of the MZ modulator 311 is branched and taken out by an optical coupler 312. The branched part of the optical output is detected by a photoelectric converter 319 such as a photodiode (hereinafter abbreviated as “PD”), and the detection signal is amplified by a buffer amplifier 320 that selectively amplifies a frequency component of f0 and inputted to a multiplier 321. The low-frequency signal that is outputted from the low-frequency oscillator 324 is also inputted to the multiplier 321. The multiplier 321 compares the phases of the signal that is inputted from the buffer amplifier 320 and the low-frequency signal that is inputted from the low-frequency oscillator 324, and outputs a signal in accordance with a phase difference.
Therefore, the multiplier 321 can detect the low-frequency signal of the predetermined frequency f0 that was superimposed by the variable gain amplifier 313.
An output signal of the multiplier 321 is inputted to one input terminal of a differential amplifier 323 via a low-pass filter (hereinafter abbreviated as “LPF”) 322 that allows passage of a frequency component of the predetermined frequency f0 or less. On the other hand, the other input terminal of the differential amplifier 323 is grounded. An output of the differential amplifier 323 is inputted to the inductor 316 of the bias T circuit as an error signal to be used for moving the operating point of the MZ modulator 311, whereby the bias value is variably controlled so as to correct the operating point.
In the MZ modulator having the above configuration, the superimposed low-frequency signal of the frequency f0 does not appear in the output light when the bias value is in the optimum state.
FIG. 21 is a waveform diagram showing an operation in a state that the operating point drifts in the MZ modulator having the above circuit configuration. Part (a) of FIG. 21 shows input/output characteristics of the MZ modulator, in which curve B represents an input/output characteristic in a case where the operating point has drifted to the high-voltage side from that of curve A and curve C represents a case where the operating point has drifted to the low-voltage side from that of curve A. Part (b) of FIG. 21 shows a waveform of an input signal and parts (c), (c1), and (c2) of FIG. 21 show waveforms of output optical signals of the respective input/output characteristics.
As shown in FIG. 21, when the operating point has drifted to the high-voltage side or the low-voltage side, low-frequency signal of the frequency f0 superimposed in output light appears with a phase that is inverted by 180° depending on the drift direction. Therefore, the bias voltage can be controlled by using a signal coming from the multiplier 321, whereby the drift of the operating point can be compensated for.
In this manner, a drift of the operating point can be compensated for by taking out a low-frequency signal from output light that has been produced by modulating input light with a modulation signal and the low-frequency signal and then comparing its phase with the phase of the original low-frequency signal. Therefore, the operating point control circuit described above can control the operating point to stabilize it in a case where input light (output light) and a modulation signal exist.
FIG. 22 is a block diagram showing a conventional optical add/drop apparatus.
As shown in FIG. 22, after wavelength-division multiplexed signal light transmitting through an optical transmission line is amplified to a predetermined light intensity, it is then entered to an OADM (optical add-drop multiplexer) node section 350 which adds/drops on the wavelength-division multiplexed signal light. Signal light beams of predetermined wavelengths are dropped by the OADM node section 350 and subjected to receiving operations in optical dropping sections 352 that are provided in the same number of signal light beams to be branched by an optical coupler 351. Signal light beams to be added by the OADM node section 350 is generated by optical addition sections 355. The optical addition sections 355 are provided in the same number of signal light beams of respective wavelengths to be added by the OADM node section 350. The added signal light beams and the signal light that has not been dropped in the OADM node section 350 are wavelength-division multiplexed, amplified, and then outputted to the optical transmission line.
In each optical addition section 355 of this optical add/drop apparatus, light that is exit from an LD 360 for generating light of a particular wavelength is amplified by an optical amplifier 361. Output light of the optical amplifier 361 is modulated by an optical modulator 362 having the above-described operating point control circuit. The modulated optical signal is amplified by an optical amplifier 363 and then entered to an optical coupler 354. The optical coupler 354 adds this optical signal to the OADM node section 350 together with optical signals of other wavelengths that have been generated by other optical addition sections 355 having the same configuration.
Incidentally, in the MZ modulator 311 shown in FIG. 20, the following problem occurs when there is a short break in which the input light entered to the MZ modulator 311 is temporarily non-existent and then recovers.
When the input light no longer exists, there is no light output to be branched by the optical coupler 312 and hence the operating point becomes indefinite. That is, in part (b) of FIG. 21, it is impossible to judge whether the bias voltage Vb is (1) 0 V or less, (2) greater than 0 V and smaller than Vp, or (3) Vp or more.
If the input light recovers in such an indefinite state, in case (2) the optimum operating point is established by the operation of the bias T circuit. However, the optimum operating point is not established in cases (1) and (3); Vb is predetermined at 0 V in case (1) and Vb is predetermined at Vp in case (3).
By these reasons, when a short break occurs in the input light that is incident on the MZ modulator 311, the optimum operating point is not necessarily obtained.
Hitherto, the above problem did not occur because the MZ modulator 311 was used in terminal stations or the like where no short breaks occur on the input light. However, where the MZ modulator 311 is used in each optical addition section 355 of the optical add/drop apparatus of FIG. 22, it is necessary to switch the wavelength of addition light to a wavelength that is not used in a wavelength-division multiplexed signal transmitting through the optical transmission line. This necessarily causes, during such wavelength switching, a state where no input light exists. Therefore, the solving the problem of being in the above indefinite state is a particularly important issue.
On the other hand, in the optical add/drop apparatus of FIG. 22, when there is no input light to the optical modulator 362, ASE (amplified spontaneous emission), which is a noise level spontaneously generated by the optical amplifiers 361 and 363, is outputted to the optical transmission line. Further, each optical addition section 355 does not always have a modulation signal to be added. When no such modulation signal exists, not only ASE but also input light that is not modulated with any modulation signals are outputted to the optical transmission line.
Further, in optical communication networks, the judgement of malfunctions occurring therein is based on the light intensity. Therefore, the malfunction cannot be judged if ASE or input light that is not modulated with a modulation signal is inputted to an optical transmission line.